Some rearchitecting of the VFS's symlink walking code. Reduces
stack usage and apparently permits us to increase the maximum hop
count from 5 to 8, although the patch doesn't actually do
that.

Implementation of request barriers for IDE and SCSI. The idea
here is that a filesystem can tag an IO request as a barrier and
the disk will not reorder writes across the barrier. It provides
additional integrity guarantees for the journalling filesystems.
The feature is enabled for reiserfs and ext3.

On reiserfs do `mount /dev/hda /wherever -o barrier=flush' or
`barrier=none'.

Although this feature has been around for a while it is new
code, and the usual cautions apply. If it munches all your files
please tell Jens and he'll type them in again for you.

The pagecache radix-tree spinlocks have gone back to rwlocks
again. It helps big SMP significantly and doesn't seem to make much
difference to small SMP (1-2% at most IIRC). It does need some more
measuring.

Added a new SATA RAID driver from 3ware. From a quick peek it
seem to need a little work yet.